Conventionally, electro-pneumatic positioners have been provided in control valves, where the valve opening of the control valve is controlled by the electro-pneumatic positioner. This electro-pneumatic positioner includes a calculating unit for calculating a deviation between a control valve opening setting value sent from a higher-level device and an actual opening value that is fed back from the control valve, to generate, as a control output, an electric signal in accordance with this deviation; an electro-pneumatic converting device for converting, into a nozzle back-pressure of a nozzle/flapper mechanism that is operated by air that is supplied through a fixed orifice from a pneumatic pressure supply source, the control output generated by the calculating unit; and a pilot relay for amplifying the nozzle back-pressure, converted by the electro-pneumatic converting unit and outputting the result, as an output pneumatic pressure, to an operating device for the control valve.
In this electro-pneumatic positioner, air from a pneumatic pressure supplying source is directed through a fixed orifice to a nozzle, strikes a flapper, and is exhausted. Fine dust (or mist) that is included within the air from the pneumatic pressure supplying source gradually accumulates in the narrow fixed orifice, which is the narrowest place in the air flow path, or in the gap between the nozzle and the flapper (the “nozzle gap”) where the amount of accumulation increases with the amount of time of operation. When the amount of accumulation exceeds a tolerable value, then the device falls into a non-operating state.
There are also fixed orifice and nozzle/flapper mechanism in electro-pneumatic converting devices that produce a control output from the deviation between a set opening value from a higher-level device and a measured value for the pneumatic pressure outputted to the operating device of the control valve are as well, and thus the same problem occurs as in the electro-pneumatic positioner set forth above.
Given this, consideration has been given to promoting maintenance prior to falling into the non-operating state, due to the accumulation of dust, through the electro-pneumatic positioner or electro-pneumatic converting device itself being aware that the amount of dust accumulated is large, and providing notification to a maintenance technician, either in the workplace or remotely.
For example, in Japanese Unexamined Patent Application Publication H-6-294401 (“JP '401”), a feedback signal and a deviation signal, or a difference between signals related thereto, are processed to produce a difference signal, and a warning is outputted to the outside if this difference signal exceeds a reference value that has been set in advance.
Moreover, in the Japanese Unexamined Patent Application Publication H11-311217 (“JP '217”), a dirt detecting nozzle/flapper is connected in parallel to the nozzle/flapper mechanism to the supply air duct in order to detect by proxy the dirt in the nozzle/flapper mechanism of the electro-pneumatic converting unit, where the nozzle back pressure of the dirt-detecting nozzle/flapper is measured, the amount of dirt in the electro-pneumatic converting unit is calculated from the nozzle back pressure that is measured, and that amount of dirt is displayed.
However, while it is possible to detect the occurrence of dirt that cannot be ignored (that is, a blockage) someplace in the fixed orifice or the nozzle/flapper mechanism using the methods set forth in JP '401 and JP '217, above, is not possible to specify which of these it was in which the dirt (blockage) occurred.
The present invention is to resolve this type of problem, and the object thereof is to provide an electro-pneumatic positioner and electro-pneumatic converting device able to specify whether the blockage has occurred in the fixed orifice or in the nozzle/flapper mechanism.